My 105s had +2 744 and +2 752 hand written and I wondered if it was something to do with the measurement plot. Can't be resonant freq direct since speced at 620Hz +/- 20Hz for 95% of production, or 650Hz +/- 70Hz total.

Yes, mine had the printed labels also. Batch numbers and item numbers, an H number with 2 sets of 2 digit numbers following, a serial number, an inspection number, an R and S label with 2 digit numbers following, and another number with a zero with a stroke through it followed by 37.5 and 39.5.

All this in addition to the 744 and 752 labels. I think they were well measured at the factory and ofter wondered what the more esoteric terms signified!

O with stroke trough it and 27 or LZ (or can be also L2)and O with stroke trough it on round blue sticker.

No234 O with stroke trough it and 38,0 hand written on white rectangular sticker on side of tweeter.

If Audiolabtowers ques for 805 being the resonant frequency, even it would be off spec, would it be possible if +1 is indicating efficiency of unit somehow? 234 can be sorting number. It would be most obvious to use resonance and efficiency when sorting T52.s.

However, was inductance used for sorting tweeters? I noticed that it does vary quite much between T52:s in my previous tread. You suggested that my meter was off spec, but I have measured several crossover coils with it and none have been off spec. T52 VC:s were measuring between 0,08 mH and 0,5 mH. Now T52:s that are farthest off the spec are working on Cantatas. It is difficult to tell how well, but I suppose they are performing 1000 % better than T33:s with old ferrofluid in P60:s.

Speakerguru mentioned that measuring VC inductance is not easy task, but at least all my VC:s have been measured with same metering setup?

Hi, interesting. It looks like there could be a difference between the theoretical spec figures and simple practical measurement as previously explained by Speakerguru. Inductance is an interesting theory. I thought the +1 or +2 might be some indication of Q of the resonant freq, but it could be something else - sensitivity band like Spendor selected - I'm just guessing?

Sensitivity is the only plausible suggestion of what the figures might be. The resonance frequency of a T52 is very low compared to the crossover frequency and can be ignored as a variable. Similarly, variations in the vc semi-inductance would be minor and not have a significant effect on speaker/crossover matching.